Re: [PATCH v3 2/3] drivers/perf: add DesignWare PCIe PMU driver

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, May 16, 2023 at 04:03:04PM +0100, Jonathan Cameron wrote:
> 
> PCI folks, Question below directed at you. Please take a look.
> +CC linux-cxl because a similar question is going to bite us shortly
> if we want CXL PMUs to work well on RP or Switch ports.
> 
> > >> +static int dwc_pcie_ras_des_discover(struct dwc_pcie_pmu_priv *priv)
> > >> +{
> > >> +    int index = 0;
> > >> +    struct pci_dev *pdev = NULL;
> > >> +    struct dwc_pcie_rp_info *rp_info;
> > >> +
> > >> +    INIT_LIST_HEAD(&priv->rp_infos);
> > >> +
> > >> +    /* Match the rootport with VSEC_RAS_DES_ID */
> > >> +    for_each_pci_dev(pdev) {  
> > > 
> > > Does the PCI layer not offer a more robust mechanism for this?
> > > (PCI fixups come to mind, but I don't actually know whether that
> > > would be a viable approach or not.)   
> > 
> > I am afraid not yet. Jonathan try to add a PMU service but it is
> > not merged into mainline.
>
> I wouldn't read much into that 'failure'.  We never persisted with
> that driver because it was for an old generation of hardware.
> Mostly the aim with that was to explore the area of PCIe PMU in
> general rather than to get the support upstream. Some of the
> counters on that hardware were too small to be of much use anyway :)
> 
> Grabbing just relevant functions..
> 
> Bjorn, we need to figure out a way forwards for this sort of case
> and I'd appreciate your input on the broad brush question of 'how
> should it be done'?
> 
> This is a case where a PCIe port (RP here) correctly has the PCIe
> class code so binds to the pcie_port driver, but has a VSEC (others
> examples use DOE, or DVSEC) that provides extended functionality.
> The referred to PCIe PMU from our older Hisilicon platforms did it
> by adding another service driver - that probably doesn't extend
> well.
> 
> The approach used here is to separately walk the PCI topology and
> register the devices.  It can 'maybe' get away with that because no
> interrupts and I assume resets have no nasty impacts on it because
> the device is fairly simple.  In general that's not going to work.
> CXL does a similar trick (which I don't much like, but too late
> now), but we've also run into the problem of how to get interrupts
> if not the main driver.

Yes, this is a real problem.  I think the "walk all PCI devices
looking for one we like" approach is terrible because it breaks a lot
of driver model assumptions (no device ID to autoload module via udev,
hotplug doesn't work, etc), but we don't have a good alternative right
now.

I think portdrv is slightly better because at least it claims the
device in the usual way and gives a way for service drivers to
register with it.  But I don't really like that either because it
created a new weird /sys/bus/pci_express hierarchy full of these
sub-devices that aren't really devices, and it doesn't solve the
module load and hotplug issues.

I would like to have portdrv be completely built into the PCI core and
not claim Root Ports or Switch Ports.  Then those devices would be
available via the usual driver model for driver loading and binding
and for hotplug.

Bjorn